1. Field
The present specification generally relates to systems and methods for producing aromatic carboxylic acids and, more specifically, to systems and methods for producing aromatic carboxylic acids from aromatic feedstocks by a liquid-phase oxidation in the presence of a potassium manganate oxidizing agent that is regenerated and recycled by the systems and during the methods.
2. Technical Background
Aromatic carboxylic acids are large-scale commodity chemicals often produced by oxidizing aromatic compounds that have been derived from petroleum fractions. Terephthalic acid, also known as 1,4-benzenedicarboxylic acid, for example, is widely produced in various degrees of purity by oxidation of p-xylene (1,4-dimethylbenzene). Billions of tons of terephthalic acid are produced in this manner annually for principal end use as a precursor to polyethylene terephthalate, a material used for clothing and plastic bottles.
On the bench scale, oxidation of aromatic compounds such as p-xylene is readily carried out by exposing the aromatic compounds to strong oxidizing agents such as permanganate (MnO4−; Mn(VII)) compounds or dichromate (Cr2O72−; Cr(VI)) compounds. The bench-scale process proves unworkable in medium-scale or large-scale industrial processes, however, because solid byproducts such as MnO2 or various chromates are formed in sizable amounts. Such solid byproducts are detrimental to the environment and, therefore, require high disposal costs if they are merely discarded as waste on the industrial scale.
Current commercial manufacturing of aromatic carboxylic acids such as terephthalic acid, for example, may involve one or more of complex catalyst systems, acidic media, or bromine sources. These chemistries tend to be corrosive, so as to necessitate the use of expensive reactors made of metals such as titanium. There remain ongoing needs for oxidation technologies for aromatic compounds that may avoid corrosive media that necessitate expensive reactors, that may exhibit high conversions to usable product, and that may avoid environmental concerns by limiting or eliminating solid waste byproducts.